Preheat your oven to 425°F (220°C).

In a skillet over medium heat, add 1 teaspoon of olive oil. Once hot, add 3 cloves of minced garlic and a pinch of red pepper flakes, stirring for about 1 minute until fragrant.

Add 5 cups of baby spinach to the skillet, stirring until wilted, which should take around 2-3 minutes.

Remove from heat and stir in 4 ounces of chopped sun-dried tomatoes and 3 ounces of crumbled feta cheese. Allow it to cool.

In a mixing bowl, whisk together 2 tablespoons of balsamic vinegar, 2 tablespoons of olive oil, 1 teaspoon of Dijon mustard, 1.5 teaspoons of minced rosemary, the zest of 1/2 lemon, 1 grated clove of garlic, 3/4 teaspoon of kosher salt, and 1/4 teaspoon of black pepper.

Cut a lengthwise slit down the center of the pork tenderloin, creating a pocket without cutting all the way through.

Open the tenderloin and pound it to an even thickness.

Spoon the spinach and feta filling into the pocket, spreading evenly, then fold the pork back over.

Secure the tenderloin with toothpicks or kitchen twine.

Place in a baking dish and drizzle the topping over it.

Bake for 25-30 minutes until the internal temperature reaches 145°F (63°C).

Remove from the oven and cover loosely with aluminum foil. Let it rest for 5-10 minutes before slicing.